1. Field of the Invention
The present invention relates to an ultra compact fingerprint input device, and more particularly to a fingerprint input device of in-finger scattering type in which a fiber optics plate constituting light transmitting means is fixed on a silicon chip bearing an image pickup element, and the interior of a finger placed on a surface of the fiber optics plate is irradiated with an infrared light, a near infrared light, a red light etc. and a light scattered from the interior of the finger is received through the fiber optics plate with the image pickup element to read the fingerprint of the finger.
2. Description of the Related Art
With the recent pervasiveness of the electronic trading and similar economic activities based on the remarkable progress in the information technologies, the necessity for electronic personal authentication is increasing for the purpose of avoiding improper use of information.
For electronic personal authentication, there have been employed various methods of inputting an image of a fingerprint. For example Japanese Patent Application Laid-open No. 2000-21780 discloses a method of emitting a near infrared light toward the interior of a finger, maintained in contact with the vicinity of a surface of a two-dimensional image pickup element and receiving a scattered light from the interior of the finger. This method allows to construct an extremely compact fingerprint input device without requiring a total reflecting prism or the like.
However, a semiconductor, including a solid image pickup element, is generally very fragile, and is easily broken by an application of a physical force or an electrostatic charge, or a deposition of moisture or salt caused by a direct contact of a finger or the like. In order to prevent such damage, light transmission means such as a fiber optics plate (hereinafter also represented as FOP) is usually adhered onto the silicon chip. The FOP is formed by adhering and cutting out a bundle of glass optical fibers, and can provide a chemical stability and strength far superior to those of the silicon chip at a suitably selected thickness.
Also as disclosed in Japanese Patent Application Laid-open No. 7-174947, it is known that a clear fingerprint image can be obtained by a suitable selection of an inclination angle α at the cutting of the FOP, as the light from a portion in contact with the fingerprint is easily transmitted while the light from other parts in contact with the air is scarcely transmitted. A prior configuration disclosed in Japanese Patent Application Laid-open No. 7-174947 is shown in FIG. 5.
Referring to FIG. 5, an FOP 3 on which a finger of a fingerprint inputting person is fixed by adhesion to an image pickup unit 1a on a silicon chip 1. An inclination angle α of the FOP 3 is about 30°, generally within a range of 20 to 40°, though an optimum value is variable depending on a refractive index of the optical fibers constituting the FOP.
The FOP is generally prepared by forming a bundle of a plurality of extremely fine optical fibers, then fixing the fibers under heat and pressure to obtain an ingot (block), cutting and polishing such ingot at a predetermined inclination angle. In order to improve the efficiency of manufacture, there is generally employed a method of at first preparing an ingot of a sufficiently large cross section, then slicing it into a plate with predetermined thickness and inclination angle, and finally cutting it into a predetermined planar dimension.
Since the FOP has an acute inclination angle of about 30° as mentioned above and also since the glass is brittle as known well, the FOP with a parallelogram cross section as shown in FIG. 5 is very easily chipped and is inconvenient for handling in the manufacture or for use in the fingerprint detection.
In case the FOP 3 is formed into a circumscribed rectangular cross section of the parallelogram as shown in FIG. 6A, there is generated a light non-transmitting area 3a not contributing to the transmission of the fingerprint image and optically constituting a shadow. A length L of such light non-transmitting area 3a is given by L=T·tan(90°-α) for an inclination angle α and a thickness T of the FOP 3, and usually becomes larger than T. In case the thickness T is selected at about 1 mm in order to withstand the pressure by the finger 10, the length L of the light non-transmitting area 3a becomes even larger.
Though it is also possible to construct such light non-transmitting area 3a as an overhanging portion protruding from the silicon chip 1 as shown in FIG. 6B, such structure is unsatisfactory in the strength and in the manufacturing process, since the FOP is subjected to a bending stress by the pressure of the finger 10.